A cooling or exhaust fan, which has a DC motor, is usually designed to rotate in only one direction. However, there is a possibility that the fan doesn't rotate in the pre-determined direction in some abnormal situation. For example, when a cooling fan, positioned at the front of a car, is facing against a strong wind during the car's running, or an exhaust fan for a kitchen is subjected to a strong wind from the outside to the inside, the fans may rotate in the reverse direction, resulting in an inability to cool or exhaust as expected.
One resolution to this problem is to install several backwind shutters beside the fan, for instance, in front of the aforesaid fan in the car or on the outside of the aforesaid fan in the kitchen, and the shutters are designed to open only in a predetermined direction. In the ordinary working state, the fan normally realizes the effect of cooling or exhausting after the shutters are blown open by the wind generated by the fan. When a contrary wind occurs which is stronger than the wind generated by the fan, the shutters are blown closed and leave the contrary wind outside to prevent the fan from being driven in the reverse direction.
On the other hand, in the non-working state, when there is a strong wind blowing in the same direction as that of the wind generated by the fan, such as a wind blows from the inside to the outside, the blades of the fan will be pressed by the strong wind while the shutters are blown open, and then the fan will be forced to rotate and create a BEMF (Back Electromotive Force), leading to the fan operating as an electric generator. This may damage the control circuit for the fan if the generated BEMF is too high.